Multiverse Madness is similar to a Firefight mode or a Horde mode. This will roughly take you about 2 hours to complete. This is easier done in co-op. After the first 10 rounds all enemies will get a x1.5 attack damage and health upgrade & from round 20 - 30 all enemies will get a x2.0 attack damage and health upgrade.
Stay in the starting area of this map (the room that is near Joes special item with the health pack special in it). This way you force all the enemies to come through the only rooms entrance. Once you either have the Acid Sprayer or the Magam Gun, stand at the top of the steps, and launch the acid or magam in front of the steps. If you get hurt in this progress, there is a health pack on the opposite side of the wall.
-Quagmire - Save enough money so you can purchase the Acid Sprayer, which will melt the enemies almost instantaneously.
-Stewie - Save enough money so you can purchase the Magma Gun, which will melt the enemies almost instantaneously.
Landmines - These will help you set up defense to protect yourself.
If playing in co-op, both players will pop the achievement. And both players should be using Quagmire & Stewie. But if either player manages to die on the last wave, and the wave is completed. That player will not get the achievement.